An illumination system has been presented in US 2005/0088760, wherein the rays reflected from a reflective object in an object plane enter the projection objective on divergent paths. In the case of an axially symmetric projection objective which has an optical axis, this means that the projection objective has a negative back focus of the entry pupil. For axially symmetric systems, this means that a positive principal ray angle γ is present at the reflective object in the object plane. In the examples presented in US 2005/0088760, the positive principal ray angle γ is less than 7° (e.g., less than 6°).
In the case of a negative back focus of the entry pupil a mirrored entry pupil in a mirrored entry pupil plane is obtained by a mirror reflection on the object plane. The mirrored entry pupil plane lies in this case to the image side of the object plane.
A positive back focus is present if the principal ray angle γ in the object plane is negative, i.e. if the principal ray of the central field point after the reflection on a reflective object in the object plane, for example on the reflective reticle, proceeds on a path that converges towards the optical axis. In the case of a positive back focus of the entry pupil, the entry pupil of the projection objective lies to the image side of the object plane and, accordingly, the mirrored entry pupil plane lies to the opposite side of the object plane.
The back focus is determined by the distance from the object plane to the point where the principal ray directed to the central field point of the illuminated field in the object plane intersects the optical axis. With a positive principal ray angle at the object or at the reticle, for example γ=8°, and with a field radius r=125 mm, the back focus SEP is obtained as SEP=−R/tan γ=−889.4 mm. In systems with a negative back focus of the entry pupil, the principal ray angle γ at the object is positive.